This invention is directed to a method of applying an environmental or bond coating applied to turbine engine assemblies and parts, such as airfoils and shrouds, using a thermal spray process, and specifically to a method of applying MCrAlY and other HVOF-applied coatings having key quality characteristics required to protect the coated parts in a high temperature, oxidative and corrosive atmosphere while permitting application of long life thermal barrier topcoats.
Many systems and improvements to turbine coatings have been set forth in the prior art for providing protection to turbine airfoils and shrouds in and near the flowpath (hot section) of a gas turbine from the combined effects of high temperatures, an oxidizing environment and hot corrosive gases. These improvements include new formulations for the materials used in the airfoils and include exotic and expensive nickel-based superalloys. Other solutions have included application of coating systems, including environmental coating systems and thermal barrier coating systems. The environmental coating systems include nickel aluminides, platinum aluminides and combinations thereof. Known processes and methods of applying the include thermal spray techniques including but not limited to low pressure plasma spray (LPPS), hyper velocity oxy-fuel (HVOF) and detonation gun (D-gun), all of which thermally spray a powder of a predetermined composition.
A multitude of improvements in such coatings and in methods of applying such coatings has been set forth that increase the life of the system, and developments in these improvements continue. In certain systems, thermal barrier coatings (TBC's) in the form of a ceramic are applied over the environmental coatings. In other systems, a bond coat such as a MCrAlYX, where M is an element selected from Ni, Go, Fe or combinations of these elements, and where X is a trace metal such as Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, is applied as an intermediary between the airfoil and the applied ceramic. The bond coat is also to improve the environmental performance of the system. The coatings which include aluminides and MCrAlX alloys can be non-brittle or brittle, depending upon whether they are comprised substantially of gamma or gamma+gamma prime phases.
Despite the many improvements in the field of applied environmental coatings, a continuing problem is that known coating methods do not provide a sufficiently thick and uniform coating on part edges, especially on acute edges such as on high pressure turbine shrouds (“HPT shrouds”) and low pressure turbine shrouds (“LPT” shrouds) and similar parts in the turbine flowpath. Application of the coating to such flowpath parts is frequently accomplished using a Hyper-Velocity OxyFuel (“HVOF”) thermal spray process, which is often robotically controlled. However, using known tooling and methods, the HVOF process tends to leave a thinner coating on the fore and aft edges of parts such as shrouds, and the coating tends to round out on the edges as it is applied. Such rounding leaves an insufficiently thick coating for proper machining of edges to the desired shape, and can result in an exposed edge, or in insufficient coating to protect the underlying edge during turbine operation.
What is needed are cost effective methods that can be employed to ensure that edges and other flowpath surfaces of blades, shrouds, and other flowpath parts are sufficiently coated so as to permit subsequent machining to provide the desired edge shape, while still providing adequate coating thickness to protect the underlying part.